A recently developed technique of tracking motion and accumulating position by correlating shifted instances of a previous image with new images of surface micro-texture, such as the fibers in a sheet of paper, offers ease of use and outstanding accuracy. Devices of these sorts are described in the incorporated patent documents. A generic term for such a technique is xe2x80x9coptical navigationxe2x80x9d, and it can reasonably be expected that it will come into ever wider use as it becomes more widely known and appreciated. Power consumption is often an issue in the types of applications that optical navigation is suited for. For example, a hand held scanner is much more attractive as a battery powered device than as one powered from the AC mains. A battery powered computer operated with a mouse that uses optical navigation is a further example.
The optical navigation technique depends upon illumination. The photosensitive elements that are used to capture the image have an xe2x80x9celectronic shutter,xe2x80x9d in that they can be continuously exposed to the illuminated image without benefit of a mechanical shutter. Ideally, they accumulate charge or otherwise transduce from light to an electrical value only in response to a signal to do so. Unfortunately, some photosensitive devices experience bias phenomena that interfere with making measurements when the light is pulsed on after being off. These devices can still be said to have an electronic shutter, but the nature of the devices dictates that the source of light be left on. There are however, types of photosensitive devices that can be disposed in arrays suitable for optical navigation, and that both have electronic shutters and are less sensitive to the bias phenomena (e.g., photo transistors, such as used in digital cameras), allowing the power savings of pulsed light operation. Our interest herein is directed to an optical navigation system having such an array of sensors.
Consequently, early optical navigation systems simply supplied the desired illumination continuously, even though it might be needed only for five or ten percent of the time. In addition, they continuously sample at a maximum rate. Ordinarily, simplicity in a complex system is a welcome property, and in this case it is not that continuous illumination and maximal sampling do not work; they do, and quite well, too. But unfortunately illumination of the surface is performed with IR (Infra Red) LED""s, and is a significant consumer of battery power, which is limited. It would be desirable if the optical navigation technique could be modified to cooperate with illumination on demand without interfering with its ability to reliably keep track of where it is. A reduction in sampling rate, where possible, would also be desirable, as it is the taking of the samples that is the major consumer of power.
An optical navigation system saves power by pulsing its surface illumination light source to provide light only when needed. The level of light may be controlled by a servo-mechanism that monitors degree of correlation data and average illumination, and that changes the controlled level at times when it is both required and safe to do so. As for creating different levels of illumination, that may be accomplished by: (1) pulsing the light on and then opening the electronic shutter for some period of time during the pulse (gating the light); (2) opening the shutter and then varying the pulse width of the light (gating the LED); (3) opening the shutter and then pulsing the LED at different intensities; and (4) a combination of (2) and (3). In addition, the velocity of the navigation circuit over the surface (or vise versa) can be determined, and in cases where it is safe to assume that limited acceleration is possible (e.g., a hand held scanner), at low velocities it is desirable to reduce the rate of data acquisition, and thereby pulse the light source less often and conserve power. The reduction can be a calculated prediction of when it is actually necessary to sample again, based on a worst case acceleration, or it can simply be a conservative estimate that involves suppressing normally scheduled pulses and honoring others.